Modifier Properties don't show up for a collection
There is no way to make the Modifiers Properties Panel show up when you have a Collection selected.
While an object like a cube, curve and such (objects like cameras and lights can't have modifiers either) can have a modifier that affects its mesh data or other data like UVs, Collections are only a placeholder with functionality (like an empty, but without origin).
They them self do not contain data that could be affected by modifiers.
The idea of using a collection to apply modifiers to all its children, is itself not bad, and I believe used in other software, but in Blender you need to add a modifier to one of the child objects and then copy the modifier to all other children in the collection/selection.
Solution:

Add the modifier(s) to one object.
Select all objects you want to get the same modifier(s). If they are all children of a collection, you can RMB to Select Objects in the Collection and all its children will be selected.
Make sure the object with the modifiers is the last selected/active one (visible by having a brighter outline in the 3D View).
Press Ctrl+C and choose Copy Modifiers, as shown below:


Now all the selected objects have the same modifiers.
Happy Blending